Meiosis

1. Meiosis Chapter 11.4

2. Chromosome Review • Let’s review… • What is a chromosome? • Condensed chromatin (DNA + proteins) only visible during cell division!! • How many chromosomes do you have in each of your cells? • 46 (23 pairs) • But wait..do any of your cells NOT have 46? • SEX CELLS!

3. Chromosome Number • What does it mean to have 23 pairs of chromosomes? • 23 chromosomes: one copy of each from mom & one from dad! • Cells that have a chromosome from each parent are DIPLOID (2N)! • Somatic cells (body cells) • “two sets”

4. Chromosome Number • Some cells don’t have a pair of each chromosome! • Gametes (sex cells) of sexually reproducing organisms • These cells are HAPLOID (N)! • “one set” • Single set of genes! • Why do they only have half the number of chromosomes? Each gamete can only contain one set of genes! They COMBINE to Become DIPLOID!

5. Chromosome Structure Review • Chromatid • One strand of a duplicated chromosome • Joined by a centromere to its sister chromatid • Sister chromatids • Two chromatids joined by a common centromere • Each carries identical genetic information • Together called a DYAD

6. Meiosis • The genetic information that we (and other sexually reproducing organisms) inherit comes from two cells: sperm and egg • Meiosis – cell division in which the chromosome number is cut in half • Gametes (sperm and egg) divide this way! • Germ cells in the testis and ovary undergo meiosis and produce gametes.

7. Meiosis: Reduces the number of chromosomes/cell by half • Occurs in two steps: • Meiosis I • Meiosis II • Meiosis I and II each have prophase, metaphase, anaphase, and telophase stages 2N = 4 N = 2

8. Meiosis 2 dyads • DNA replication occurs during interphase before the beginning of meiosis I but not before meiosis II • Chromosomes will associate with the other member of its pair during Meiosis • Homologous chromosomes (also called a TETRAD) 1 tetrad

9. Meiosis • Meiosis I is a reductionaldivision: the number of centromeres is reduced by half after this division • Meiosis II is an equational division: the number of centromeres remains equal after this division

10. Meiosis 1: Prophase 1 • Prophase I • Chromosomes become visible • Homologous chromosomes pair up to form tetrads • consumes 90% of the time for meiosis • Crossing Over occurs • Results in Genetic Variation • New allele combinations!

11. Two major sources of genetic variation in Meiosis I • CrossingOver • Creates new combinations of mom and dad’s alleles • Think about chromosomal mutations! • IndependentAssortment

12. Metaphase 1 • As prophase I ends, tetrads attach to spindle fibers • Tetrads line up at center of cell

13. Anaphase 1 • Dyads pulled toward opposite poles by spindle fibers • Disjunction: separation of chromosomes • Nondisjunction leads to polyploidy! (extra chromosomes) • Note the exchange of information between paternal and maternal chromosomes

14. Telophase 1 • Separated chromosomes cluster at opposite ends of cell • Nuclear membrane forms around each cluster • Cytokinesis follows and forms 2 new cells

16. This was a reductional division: number of centromeres reduced per cell. Prophase I: 4 centromeres, therefore 4 chromosomes Prophase II: 2 centromeres, therefore 2 chromosomes/cell

17. Results of Meiosis 1 • Two daughter cells • Neither with two complete sets of chromosomes (haploid) • Sets have been shuffled and independently assorted • Chromosomes differ between each other and the original cell

18. Meiosis II • Two cells enter second meiotic division • Neither cell goes through DNA replication prior to this division!

19. Prophase II • Chromosomes (dyads) become visible • Do not form tetrads because they are already separated from homologous pair!

20. Metaphase II & Anaphase II • Chromosomes (dyads) attach to spindle fibers and line up in center of cell • Remember…they aren’t paired with another chromosome! • Anaphase II – chromatids (monads) separate from each other at the centromere • move to opposite poles!

21. Telophase II TelophaseII & Cytokinesis • Four genetically different haploid cells produced (N) • Each monad may be an entirely new combination of maternal and paternal genetic information

22. Meiosis II

23. Meiosis Review • Meiosis II • Prophase II: Dyads reappear, no tetrads! • Metaphase II: Dyads line up in middle • Anaphase II: Monads pulled apart (separated at centromere) • Telophase II: four new genetically different haploid daughter cells with monads Meiosis I • Prophase I: tetrads form (homologous chromosomes pair), crossing over occurs • Metaphase I: tetrads line up in middle • Anaphase I: Dyads pulled apart • Telophase I: two new genetically different haploid daughter cells with dyads

24. Meiosis: Formation of Gametes • Meiosis results in two kinds of haploid, sexual gametes • Males produce sperm • Females produce eggs (usually only one of the four egg cells is used!) • Sperm fertilizes egg to produce 2N zygote! • Goes through mitosis & cell specialization to form a new organism!

25. Mitosis vs. Meiosis: a Comparison • Both preceded by DNA replication • Both are methods of cell division • Both include Prophase, Metaphase, Anaphase, and Telophase • Both are followed by cytokinesis Mitosis vs. Meiosis

26. Contrasting Meiosis & Mitosis Mitosis • Each daughter cell receives a complete set of chromosomes • Less genetic diversity • Doesn’t change the chromosome number of the original cell • Single cell division • Two genetically identical diploid daughter cells • Asexual reproduction • Makes somatic cells Meiosis • Two alleles for each gene segregated and end up in different cells • Greater variety of possible gene combinations • Reduces the chromosome number by half • Two rounds of cell division • Four genetically different haploid daughter cells • Sexual reproduction • Makes gametes

28. Gene Linkage • Genes on different chromosomes assort independently • What about genes on the same chromosome? • Tend to be linked! • Chromosomes assort independently, but typically genes on the same chromosome are inherited together • Especially when close together! • Crossing over causes some genes on the same chromosome to assort independently

29. Gene Maps • Frequency of crossing-over between genes during meiosis is used to determine genes’ locations • Farther apart, more likely that crossing over occurs between them • Close together, crossovers rare • Use frequency of crossing over to determine distances from each other and map genes’ locations on chromosomes!